1. Field of the Invention
The present invention relates to a digital copier, printer, facsimile apparatus or similar image forming apparatus and, more particularly, to an image forming apparatus allowing the colors of an image to be adjusted.
2. Discussion of the Background
It is a common practice with a digital image forming apparatus to use a look-up table (LUT) for correcting the output characteristic of a printer included in the apparatus or for enhancing a particular density range. Generally, the apparatus includes image reading or scanning means, image processing means, image writing means, and image processing means. The LUT is included in the image processing means and allows an image signal input from the image reading means to the processing means to be transformed and then output to the image writing means as an output image signal.
The problem with the LUT is that because it reflects the output characteristic of the printer as to image density, sufficient correction is not achievable when the characteristic of the printer varies due to aging or contamination. In light of this, so-called process control has customarily been executed within the apparatus. For the process control, a plurality of patterns each having a particular density are formed on a photoconductive drum, transfer body or similar image carrier. An optical sensor senses light reflected from or transmitted through each of the patterns. Then, a charge potential, bias for development and amount of laser exposure are varied on the basis of the output of the sensor, or a tonality correction table is varied. This process control is automatically executed within the apparatus and does not need any manual operation. However, because the optical sensor is not sensitive to a high density portion where a great amount of toner is deposited, it allows correction to be executed only in a low density portion to a medium density portion. Moreover, the process control cannot cope with the amount of transferred toner varying due to the aging of an image transfer section and the fixing ability of a fixing section also varying due to aging.
It has also been customary to transfer a pattern image from an image carrier to a recording medium, fix it on the medium, and cause a scanner to read it. This kind of scheme allows a tonality correction table to be selected or generated on the basis of data output from the scanner, or allows color transform coefficients and RGB (Red, Green, Blue) to YMCK (Yellow, Magenta, Cyan, Black) color transform tables to be generated. Although this approach requires manual operation, e.g., requires a person to lay the recording medium driven out of the apparatus on a glass platen, it allows correction to be effected with a high density portion where a great amount of toner is deposited. In addition, correction can be made against the aging of the image transfer section and the varying ability of the fixing section.
Japanese Patent Laid-Open publication No. 5-114962 proposes a correction method using a test pattern loaded in the apparatus itself. After image forming means built in the apparatus has formed an image representative of the test pattern on a recording medium, the recording medium is driven out of the apparatus. Then, image reading means also built in the apparatus automatically reads the image of the medium. An image signal transform table is corrected on the basis of an image signal output from the image reading means. Such a self-testing arrangement is capable of reflecting the momentary variation of the characteristic ascribable to, e.g., the deterioration of the image forming means. Therefore, despite the varying characteristic of, e.g., the image forming means, the transform table can be maintained in its optimal condition, insuring high image quality.
However, the conventional approaches described above have some problems yet to be solved, as follows.
(1) To generate the tonality correction table, use is made of reference data stored in a ROM (Read Only Memory) or a RAM (Random Access Memory). The reference data or target data are implemented as a combination of values input to the table and target values. The target values are each associated with a value produced by electrostatically forming a latent image on a photoconductive element with a preselected laser output, developing the latent image, transferring the resulting toner image to a recording medium, and reading the toner image with a scanner. Toner included in a developer bears a great amount of charge in its initial condition or in a low humidity environment. In this condition, it is likely that the toner fails to deposit on the photoconductive element or image carrier in a sufficient amount. Then, the actual image density (particularly the maximum density) on the medium undergone fixation is short of the target image density represented by the reference data. As a result, the table generated has its high density portion smeared out. Although the target image density can be faithfully reproduced up to the maximum density which the printer can output, only a constant image density is available with higher target densities and causes tonality information or the details of an image to be lost.
(2) After the recording medium has been laid on the glass platen, a cover plate or an ADF (Automatic Document Feeder) is used to cover the medium from the above. Output of the scanner varies for the same recording medium, depending on which of the cover plate and ADF overlies the medium. As a result, the amount of correction and therefore the result of the correction table varies. Such a difference stems from the difference in reflectance between the surfaces pressing the document from above. Specifically, light issuing from a halogen lamp included in the scanner is partly reflected from the medium, reflected by a first to a third mirror, and then incident to a CCD (Charge Coupled Device) image sensor. At the same time, the light is partly transmitted through the medium, reflected from the rear of the document, again transmitted through the medium, reflected by the first to third mirrors, and then incident to the image sensor. When the rear of the medium or document has a high reflectance, a great amount of light is incident to the image sensor, so that the density of the image is determined to be low. On the other hand, when the rear of the document has a low reflectance, the amount of light incident to the image sensor decreases, so that the density of the document is determined to be high. While the cover plate is implemented as a plate formed of plastics or coated with vinyl, the ADF usually conveys the document with a conveyor belt. The cover plate and belt each has a particular surface reflectance, and each eventually results in different data despite that the toner pattern is the same. Consequently, the correction table selected, and therefore tonality to be output on a recording medium, differs from the case using the cover plate to the case using the ADF. This problem is particularly serious when an image generated by a computer is to be printed on a recording medium. An image generated by a computer must be reproduced with the same tonality without regard to which of the cover plate and ADF is used.
Another issue is the kind of the recording medium, e.g., a thin paper, thick paper, recycled paper, or paper with a high degree of whiteness. Even when images are printed by the same printer at the same period and even if the developing characteristic of the printer is constant, the data output from the image sensor differs due to the difference in the amount of reflection or that of transmission ascribable to the kind of the recording medium. As a result, the developing characteristic differs in the apparent sense, causing an inadequate correction table to be used.
It is difficult to compute the contribution (ratio) of background data contained in the scanner output m ore accurately due to the difference between scanners and irregularity in adjustment (within an allowable range). Therefore, the result of automatic adjustment differs from one machine to another machine.